1. Field of the Invention
The present invention relates to cathode ray tubes, and more particularly, to a cathode ray tube (CRT) having an improved electron gun installed at the neck portion of the CRT, for exciting a fluorescent film.
2. Description of the Related Art
There are various types of CRTs depending on functional characteristics such as projectors, oscilloscopes, monitors, TVs or the like. An example of these CRTs is shown in FIG. 1.
As shown in FIG. 1, a cathode ray tube includes a panel 12 on which a fluorescent film 11 is formed, and a funnel 15, sealed to the panel 12, having a neck portion 13 in which an electron gun 20 is mounted. A deflection yoke 16 for deflecting an electron beam emitted from the electron gun 20 is installed on the cone portion of the neck portion 13. Also, a velocity modulator (VM) 17 to which a differentiation value of an image signal is applied is installed on the outer circumferential surface of the neck portion 13 to control the velocity of deflection of an electron beam by the deflection yoke 16. In this CRT, a shadow mask frame assembly 18 is installed within the panel. However, this is not necessary in the case of monochrome CRTs used in projectors, oscilloscopes, or the like. The panel 12 and the funnel 15 can be formed in a body.
There are a variety of electron guns, each installed in a neck portion, depending on the type of CRTs for forming monochrome images or color images, the arrangement of electrodes constituting an electron gun, and the state of a voltage applied to each of the electrodes. An example of these electron guns is shown in FIG. 2.
The electron gun of FIG. 2 is disclosed in U.S. Pat. No. 4,904,898, which includes a cathode 21 for emitting thermoelectrons, a control electrode 22, a screen electrode 23, upper and lower focusing electrodes 24 and 25 sequentially installed adjacent to the screen electrode 23, and a final accelerating electrode 26 surrounding the end of the upper focusing electrode 25. Embedding portions (not shown) are formed on both sides of each of the cathode 12 and the aforementioned electrodes forming the electron gun 20, and fixed by a pair of bead glasses 27.
In the CRT having this electron gun 20, a beam of electrons emitted from the cathode of the electron gun is focused and accelerated while passing through an electron lense formed between the electrodes forming the electron gun. The focused and accelerated electron beam is selectively deflected by a deflection yoke according to positions of a fluorescent film scanned, and then excites the fluorescent film. During this process, in order to more clearly show the difference between a bright area and a dark area of an image formed by excitation of the fluorescent film, a two-pole coil of the VM 17 is provided with a current which is proportional to the secondary differentiation value of each image signal, and accordingly the deflection rate of an electron beam deflected by the deflection yoke 16 at a bright area of an image and a deflection rate at a dark area thereof is adjusted, whereby the contrast of an image is improved. This method is achieved by controlling an instantaneous scanning speed of an electron beam using the two-pole coil of the VM 17 installed in the same direction as the direction of the horizontal deflection magnetic field of the deflection yoke 16.
However, the focusing electrode of the electron gun installed at a position corresponding to a position where the VM 17 is installed has a cylindrical shape, such that the electrode generates an eddy current due to a high frequency current generated by the VM 17. This eddy current causes an inverse magnetic field because of a magnetic field generated by a coil, which deteriorates the deflection sensitivity of an electron beam against the current of the VM 17. Consequently, the deterioration in the deflection of an electron beam degrades the contrast of an image.
To solve the above problem, an objective of the present invention is to provide a cathode ray tube which can prevent a deflection sensitivity from being deteriorated due to an eddy current generated because of a velocity modulator
Another objective of the present invention is to provide a cathode ray tube which can prevent a deterioration in the contrast of an image.
To achieve the above objectives, the present invention provides a cathode ray tube including: a bulb having a screen on which a fluorescent film is formed, and a funnel portion having a neck portion on the side opposite to the screen; an electron gun mounted in the neck portion of the bulb, the electron gun having a cathode forming a triode portion, a control electrode, a screen electrode, a plurality of focusing electrodes sequentially aligned from the screen electrode, and a final accelerating electrode which is installed adjacent to a focusing electrode the farthest away from the cathode or protects a predetermined width of the circumferential surface of the focusing electrode; a deflection yoke mounted on the bulb, for deflecting an electron beam emitted from an electron gun; a velocity modulator installed around the neck portion; and an eddy current generation preventing means installed on an electrode of the electrode gun, which corresponds to the velocity modulator, for preventing generation of an eddy current by the velocity modulator.
In the cathode ray tube, the eddy current generation preventing means has a plurality of slots formed along the outer circumferential surface of an electrode, which is opposite to the velocity modulator, in the lengthwise direction or in the circumferential direction.
Also, the eddy current generation preventing means is an electrode that is opposite to the velocity modulator, the focusing electrode formed by coaxially adjoining a plurality of rings to each other using an insulating material or a resistive material.